Aluminum and its alloys have been widely used in a variety of industrial and household applications in sheet forms or as strips, bars, rods, tubes, structural members, household appliances and utensils, hardware and a host of other articles. As mentioned in U.S. Pat. No. 2,941,930, there are numerous outlets for aluminum articles and its alloys for such uses as ornamental wall panels for inside or outside of various structures, restaurant furnishings, art objects and several other applications.
Because of its low density and tendency toward corrosion, it is necessary to provide aluminum articles with a suitable coating in order to impart structural strength and integrity thereto and to protect them against corrosion and environmental degradation. In the past, the metal surfaces were often painted or enameled in order to protect them against the action of the elements. However, painting and enameling do not provide the metal surfaces with satisfactory protection because they are basically organic coatings and tend to degrade at elevated temperatures. Moreover, these coatings usually adhere poorly to the metal surfaces, particularly when subjected to different temperature cycles.
In order to provide improved protection for aluminum and its alloys, the metal surfaces have been anodized using various electrolytic solutions. While anodization of aluminum affords the surface of the metal greater protection against corrosion than has hitherto been obtained by painting or enameling, still the resulting coated articles have not been entirely satisfactory because of inadequate resistance against corrosion by acids and alkalis. Moreover, the coatings imparted to the metal by known electrodeposition techniques often lack the desired degree of hardness, durability, smoothness, adherence to the metal surface and the imperviousness required to meet the ever-increasing industrial and household demands. Frequently, the coated aluminum articles have been unsatisfactory for use as decorative articles because of the poor quality or appearance of their surfaces.
There are numerous patents which deal with anodization of aluminum metal and its alloys. See, e.g., U.S. Pat. No. 4,659,440 and the patents cited therein. A review of the prior art patents illustrates the significant role of the electrolytic solution used in the anodizing process in order to provide aluminum and its alloys with the desired coatings. Thus, the nature and properties of the coatings formed on aluminum and its alloys depend, to a great extent, on the composition of the anodic bath (electrolytic solution). Other parameters such as the conditions used during the electrodeposition process also contribute to the nature and quality of the coating. These factors were recognized by the present inventor and discussed in his U.S. Pat. No. 4,082,626 and later in his aforementioned U.S. Pat. No. 4,659,440.
As described in said U.S. Pat. No. 4,082,626, a rectifier metal, (e.g., aluminum) is anodized in an electrolytic solution consisting of a relatively pure potassium silicate at concentrations exceeding the potassium silicate concentrations theretofore employed. The process comprised immersing the rectifier metal (anode) in the electrolytic solution, immersing a second metal in said solution, said second metal serving as the cathode, imposing a voltage potential across the anode and the cathode and causing an electric current to flow therebetween until a visible spark is discharged at the surface of the rectifier metal, increasing the voltage potential to about 300 volts and maintaining this potential at approximately the same level until the desired coating thickness is deposited on the surface of the rectifier metal. While the resulting coating exhibits more desirable qualities than the coatings obtained by the prior art anodizing methods, they still do not fulfill the stringent demands of various industrial and household requirements. In addition, the surface finish of the metal is not entirely satisfactory for decorative applications of the coated metallic article.
In his later U.S. Pat. No. 4,659,440, the present inventor describes the use of a different electrolytic solution for anodizing aluminum and its alloys. It consists essentially of an aqueous solution containing an alkali metal silicate, a peroxide, a water-soluble carboxylic group-containing organic acid and a water-soluble fluoride. Where the coated article is intended to be used for decorative purposes, a small amount of a vanadium compound is included in the electrolytic solution in order to impart color to the resulting coating. The vanadium compounds used to impart the desired color to the coatings include sodium vanadate (Na.sub.3 VO.sub.4), hypovanadate [M.sub.2 (CV.sub.4 O.sub.9)].H.sub.2 O, e.g., sodium pyrovanadate (Na.sub.2 V.sub.2 O.sub.7) and potassium metavanadate (KVO.sub.3), and vanadium fluorides such as vanadium trifluoride (VF.sub.3.H.sub.2 O), vanadium tetrafluoride (VF.sub.3) and vanadium pentafluoride (VF.sub.4).
In the method described in the aforementioned U.S. Pat. No. 4,659,440, the aluminum article serving as the anode, and another metal serving as the cathode, are immersed in the electrolytic solution and a "voltage shock" is applied between these two electrodes. This voltage shock is quickly raised to about 300 volts within 2 to 10 seconds, and thereafter the voltage is raised gradually to about 450 volts within a few minutes until the desired coating thickness is formed. The coating produced in accordance with the method described in said patent is more uniform and homogeneous and less pervious than the coatings produced by the method described in the earlier U.S. Pat. No. 4,082,626. In the latter patent the aluminum surface is coated with a pure silicate compound, i.e., sodium silicate or potassium silicate, whereas in the former patent the coating also includes some vanadium compound.